1. Field of the Invention
The present invention relates to a workpiece holder for rotary grinding machines for grinding semiconductor wafers, having a rotatable work surface which points toward the rotating grinding tool and on which the semiconductor wafer to be machined is laid, and having means on which the workpiece holder is supported axially. In addition, the invention relates to a method of positioning the workpiece holder.
2. The Prior Art
In the grinding of the surfaces of the semiconductor wafers, considerable importance is given to the relative spatial positions of the axis of rotation of the workpiece holder and the axis of rotation of the grinding tool of the rotary grinding machine. It may, for example, be expedient to hold the axis of rotation of the grinding tool and the axis of rotation of the workpiece holder inclined toward or against one another so as to enclose a defined angle, in order to give a special geometry, for example, a curved side face, to the semiconductor wafer through the grinding.
The usual aim is to remove by the grinding process a layer of uniform thickness from one of the side faces of the semiconductor wafer which is to be machined. The prerequisite for achieving this aim is that the work surface of the workpiece holder and the grinding plane of the grinding tool lie parallel to one another. In order to meet this condition, the axis of rotation of the workpiece holder and the axis of rotation of the grinding tool must have parallel orientations.
During the grinding of a semiconductor wafer, the positions of the axes can change because of temperature influences and prevailing process forces. Means must therefore be provided for restoring the desired relative positions of the two axes.
This was done hitherto by purposeful variation of the orientation of the axis of rotation of the grinding tool by turning the spindle of the grinding tool about a defined center of rotation, or deforming its suspension. The orientation of the axis of rotation of the rigidly mounted workpiece holder is not changed in this case. This procedure is complicated and time-consuming and, in view of the ever stricter demands on the geometry of the ground semiconductor wafers, leads to insufficiently accurate results.